Equivalents to: subject_prefix

Previous versions of Graylog allowed default values for:
transport_email_subject_prefix = XXX
transport_email_from_email = bob@bob.com
transport_email_from_name = [Graylog Bob]

Later versions removed this feature meaning it is programmatically difficult to provide indicators are email 'routing' when you run multiple environments and have different names/ status and roles for those environments.

Currently: "Graylog alert for stream: ${stream.title}: ${check_result.resultDescription}" provides nothing in the subject to identify the origin.

If "Graylog alert for stream" were the default transport_email_subject_prefix, then I could alter it to match whet we require on build.

The same applies for 'Sender' - default it to 'graylog@example.org' but better if I could set a default for the email and 'name' using transport_email_from_email/ transport_email_from_name.

## Expected Behavior

## Current Behavior

## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

* Graylog Version: 2.2.0 beta 6
* Elasticsearch Version: n/a
* MongoDB Version: n/a
* Operating System: AWS Linux
* Browser version: n/a